Add ON COMMIT behavior to multi-shard temp. tables

Data for multi-shard queries is pulled to the master node for final computation before the query returns. Temporary tables used to store this data should not persist past the end of the multi-shard queries; instead, they were persisting until the end of the session. This change gives these temporary tables `ON COMMIT DROP` behavior, which ensures they are destroyed as soon as the current query commits.

Fixes #32.